It is generally desired in the petroleum industry to convert heavy hydrocarbon oil, that is petroleum fractions having an atmospheric boiling point above about 565.degree. C. (1050.degree. F.), into lighter hydrocarbons which have higher economic value. In addition, the petroleum industry continues to desire a process that can convert heavy whole petroleum crude oil to lighter crude oil which has a substantially reduced amount of heavy hydrocarbon oil content. Other advantages sought through the treatment of heavy hydrocarbon oil, heavy whole petroleum crude oil and other similar feeds, particularly high boiling petroleum refinery residues, include hydrodesulfurization (HDS), hydrogenitrogenation (HDN), carbon residue reduction (CRR), hydrodemetallation (HDM) and sediment reduction.
Hydroconversion processes, also known and referred to herein as hydrocracking, achieve the above noted goals by reacting the feed oil with hydrogen gas in the presence of a heterogeneous transition metal catalyst. The heterogeneous transition metal catalyst is typically supported on high surface area refractory oxides such as alumina, silica, alumino-silicates, and others which should be known to one skilled in the art. Such catalyst supports have complex surface pore structure which may include pores that are relatively small in diameter (i.e. micropores) and pores that are relatively large in diameter (i.e. macropores) which effect the reaction characteristics of the catalyst. A considerable amount of research into changing the properties of hydroconversion catalysts by modifying the pore sizes, pore size distribution, pore size ratios and other aspects of the catalyst surface has resulted in the achievement of many of the aforementioned goals of hydroconversion.
An excellent example of such achievements is disclosed in U.S. Pat. No. 5,435,908 Nelson et al. in which a supported catalyst achieves good levels of hydroconversion of heavy hydrocarbon feeds to products having an atmospheric boiling point less than 538.degree. C. (1000.degree. F.). Simultaneously, the catalyst and process disclosed produces a liquid having an atmospheric boiling point greater than 343.degree. C. (650.degree. F.) with a low sediment content and a product having an atmospheric boiling point greater than 538.degree. C. (1000.degree. F.) having a low sulfur content. The catalyst includes a Group VIII non-noble metal oxide and a Group VI-B metal oxide supported on alumina. The alumina support is characterized as having a total Surface Area of 150-240 m.sup.2 /g, a Total Pore volume (TPV) of 0.7 to 0.98, and a Pore Diameter Distribution in which .ltoreq.20% of the TPV is present as primary micropores having diameters less than or equal to 100 .ANG., at least about 34% of the TPV is present as secondary micropores having diameters from about 100 .ANG. to 200 .ANG. and about 26% to 46% of the TPV is present as macropores having diameters greater than 200 .ANG..
Another method to substantially achieve some of the above noted goals of the hydroconversion of heavy oil feeds is disclosed in U.S. Pat. No. 5,108,581 Aldrich et al. As is disclosed by this reference, a dispersible or decomposable catalyst precursor along with hydrogen gas, preferably containing hydrogen sulfide, is added to the heavy oil feed and the mixture heated under pressure to form a catalyst concentrate. This catalyst concentrate is then added to the bulk of the heavy oil feed which is introduced into a hydroconversion reactor. Suitable conditions for the formation of the catalyst concentration include temperatures of at least 260.degree. C. (500.degree. F.) and elevated pressure from 170 kPa (10 psig) to 13,890 kPa (2000 psig) with exemplary conditions being 380.degree. C. (716.degree. F.) and 9,754 kPa (1400 psig). As is taught by the disclosure, the goal of such conditions is to decompose the catalyst precursor so as to form solid catalyst particles dispersed in the hydrocarbon oil of the catalyst concentrate before it is mixed with the bulk of the heavy feed oil in the hydroconversion reactor.
Despite such advances, the hydroconversion process of heavy hydrocarbon oil requires elevated reactor temperatures (e.g. greater than 315.degree. C. (600.degree. F.)) and high pressures (e.g. above 13,890 kPa (2000 psig)) of hydrogen containing gas. Due to the combination of elevated temperature and high pressures of hydrogen gas, the costs of building and operating a hydroconversion reactor are considerable. One way to reduce these costs and to improve safety of the reactor is to lower the reactor pressure. It is well known in the art that operating a hydroconversion reaction at pressures below 13,890 kPa (2000 psig)) causes the formation of intractable residues in the reactor and high levels of sediment in the product stream. The collection of residues and other sediments in the reactor and other process systems creates reactor conditions that are unpredictable and unstable. If this is to be avoided, frequent reactor shutdown and cleaning is required which causes loss of production because the reactor is not "on-line". Clearly unstable and unpredictable reaction conditions are not desirable from a product quality point of view, from a reactor operations point of view or more importantly from a safety point of view. Thus there remains an unmet need in the petroleum industry for a stable hydroconversion process for heavy hydrocarbon oil, heavy whole petroleum crude and heavy refinery residues that yield lighter hydrocarbons under pressure below 13,890 kPa (2000 psig).